Radio interference is a major cause of performance degradation in wireless radio systems. In order to mitigate radio interference and enhance performance of the system, wireless radio systems have adopted Inter-Cell Interference Coordination (ICIC) schemes in frequency domain and/or time domain.
With regard to frequency domain, ICIC relates to spatial reuse of the frequency spectrum and transmission power at network nodes (e.g. base stations). Current methods include:                Full frequency reuse, e.g. the basic operating mode of the LTE system, in which each base station uses the entire frequency spectrum with uniform power distributed across the system bandwidth, thereby creating strong interference to cell edge users.        Hard frequency reuse, used e.g. in GSM and LTE Rel 8-9, in which each base station operates in one out of a set of non-overlapping portions of the available frequency spectrum in such a way that neighboring base stations do not use the same set of frequencies. While this mitigates the interference at the cell-edge, the overall spectral efficiency is reduced by a factor proportional to the frequency reuse factor.        Fractional frequency reuse in which the available frequency spectrum is divided into two portions. One portion is common to all base stations and is used for scheduling cell-center users. A second portion is divided among base stations in a hard frequency reuse manner and used to schedule transmission to/from cell-edge users.        Soft frequency reuse enables base stations to transmit in the entire frequency spectrum with different power levels: higher transmission power in the portion of the spectrum where cell-edge users are scheduled; lower transmission power in the portion of spectrum where cell-center users are scheduled.        
With regard to the time domain, ICIC consists of muting the transmission from a base station in certain time resources to mitigate interference exhibited by mobile stations served by other base stations. For example, in LTE heterogeneous networks a macro base station (eNodeB) configures almost blank subframe (ABS) patterns to reduce interference for transmissions of pico-cells within the macro base station coverage area. In this case downlink subframes are configured where only necessary signals to avoid radio link failure or to maintain backward compatibility are transmitted, such as common reference signals, primary and secondary synchronization signals (PSS/SSS), physical broadcast channel (PBCH), SIB-1 and paging with their associated PDCCH. User data is not transmitted.
Almost blank subframes (ABS) was adopted in LTE Rel-10 to mitigate interference in heterogeneous deployments and comprise time-domain muting (TDM) patterns of data transmission in downlink subframes. The muting pattern of the aggressor cell (i.e., the macro-eNodeB) is signalled over the X2 interface to the victim cells, i.e. pico-eNodeBs within the macro-eNodeB coverage area, so that victim cells can schedule communication with users suffering strong interference from the aggressor cell in the ABS subframes. The time domain muting patterns are configured semi-statically by means of bitmaps representing up to four radio frames.
Mobile (user) nodes in the coverage area of the victim cell are configured to perform channel quality (CSI) measurements in correspondence of both ABS and non-ABS resources so as to enable the serving cell to determine whether or not the mobile station is affected by strong interference from the aggressor cell.
Another method to mitigate interference in the downlink of a cellular radio system is by utilizing transmission beamforming. In essence, in a multi-antenna system a base station may fine-tune the transmission of a downlink radio signal into a narrow beam in the direction of the intended receiver by means of transmission precoding thereby reducing the interference caused to other user nodes in the cell coverage area.
Mobile devices (user nodes) can be arranged to determine whether being interfered by a neighboring cell. An interference condition may occur upon detection that a quality of a neighboring cell becomes better than a quality of the serving cell by a threshold or for a certain period.
The above concept of ABS has also been used in transmission beam coordination. In particular, it provides a method for beam coordination between an aggressor base station and a victim base station, where the interference created by a beam transmitted by an aggressor base station is measured by mobile stations in the served area of a victim base station. Depending on the measured interference, a restriction of use of radio resources is determined for the aggressor base station, which is used by the victim base station to schedule users in restricted radio resources as in the ABS case.
A mobile station in the victim cell reports which beams are deemed as interfering and eventually a corresponding channel quality measure. Based on this information, the victim cell performs a ranking of interfering beams, and determines a scheduling of the served users based on muted/restricted resources as signaled by an aggressor cell. The aggressor cell determines a restriction of usage of radio resources based on the ranking of interfering beams at the victim cell.